What does this mystery hum sound like? According to Dr. Glen MacPherson:

The classic description is that it sounds like there is a truck idling outside your home. For some people, it is a deep and distant droning bass tone. Some people perceive the sound as a rumbling noise. The sound is louder indoors than outdoors, and louder late at night than during the afternoon. It can suddenly appear or dissappear (sic) for days or months.

For more information about the hum, go to MacPherson’s appropriately named website, The Hum.